There are many known advantages to the gasification of solid carbonaceous materials at superatmospheric gasification pressures. However, a major limitation in the operation of a pressurized gasifier has been the problem of feeding the solid carbonaceous material to the gasifier in a reliable, inexpensive, and efficient manner. Heretofore, feeding systems for the carbonaceous materials to the gasifier have generally involved the use of lock hoppers or sophisticated and complex mechanical feeding devices.
Lock hoppers involve the use of a compressible gas, such as nitrogen, to alternatively pressurize and depressurize a plurality of vessels to that the coal is introduced to the system effectively at atmospheric pressure. However, lock hoppers suffer major disadvantages which include high energy for compression of the gas, large volumes for the lock hopper vessels, control problems, and erosion of valves due to dust which interferes with the valve sealing surfaces.
Mechanical coal feeders, such as piston feeders, extruders, and centrifugal coal pumps, demand very close tolerances inasmuch as these feeders must actually pump the solid carbonaceous materials across a high pressure differential. As a result, mechanical feeders are often characterized by high expense, poor reliability, and a very high inefficiency due to friction. In addition, since the solid carbonaceous materials are abrasive, mechanical feeders display a great deal of wear and demand frequent overhauls.
A development which has improved the problems associated with obtaining the close tolerances in a mechanical coal feeder involves slurrying the carbonaceous material (ordinarily coal particles) with water so that this slurry may be pumped by relatively conventional methods. For example, in the well-known Texaco coal gasification process, coal is slurried with water and the slurry is then pumped to a superatmospheric gasifier, such that the water serves the additional purpose of affording an entrainment medium for introducing the coal into the gasifier burners. There are, however, a number of major disadvantages to this method. First of all, there is a big density difference between water (spg. 1) and coal particles (spg. 1.2-1.9). This necessitates maintaining sufficient velocity in the slurry pumping and transport system so that coal does not settle out of suspension. Such velocities can induce a great deal of wear which demands frequent maintenance. Secondly, water has a high latent heat of vaporization, and as copious water is added to the gasifier, much of the coal's energy is wasted in the evaporation of this water. Additionally, as increased water is added to the gasifier a good deal of the coal's energy is consumed in forming hydrogen from water, and in many applications this hydrogen may not be as directly useful as the other products of gasification, such as carbon monoxide. Evaporating the water from the slurry after it is pumped, but before the coal enters the gasifier, is judged not to be practical in view of the high latent heat of vaporization of the water and the high temperatures required to conduct this vaporization at elevated pressures. Recently it has been suggested that methanol could be used alternatively to water for slurrying of the coal. Methanol, however, still has a high latent heat of vaporization, although this latent heat is about half that of water. Moreover, methanol, although producable from the gases made in the gasifier, is a product considered too valuable for addition or recycle to the gasifier.